1. Field of the Invention
The present invention relates to an optical transmitter that transmits pulsed multivalue phase-modulated light, and more specifically, relates to an optical transmitter capable of compensating a phase shift due to a temperature change or the like between drive signals, respectively applied to a plurality of serially connected light modulation sections.
2. Related Art
Recently, demand for the introduction of a next generation 40 Gbit/s optical transmission system has increased, and transmission distance and frequency efficiency equivalent to those of a 10 Gbit/s system are also required. As a means for realizing this, research and development of Return to Zero-Differential Phase Shift Keying (RZ-DPSK) modulation or Carrier Suppressed Return to Zero-Differential Phase Shift Keying (CSRZ-DPSK) modulation, which are excellent in optical signal-to-noise ratio (OSNR) resistance and nonlinearity resistance as compared with a Non Return to Zero (NRZ) modulation format conventionally applied to the 10 Gbit/s systems or below, have been vigorously performed (for example, refer to T. Hoshida et al., “Optimal 40 Gb/s Modulation Formats for Spectrally Efficient Long-Haul DWDM Systems”, Journal of Lightwave Technology, Vol. 20, No. 12, pp. 1989-1996, December 2002, and O. Vassilieva et al., “Non-Linear Tolerant and Spectrally Efficient 86 Gbit/s RZ-DQPSK Format for a System Upgrade”, OFC 2003, ThE7, 2003). Moreover, in addition to the above described modulation format, research and development of a phase modulation format such as Return to Zero-Differential Quadrature Phase-Shift Keying (RZ-DQPSK) modulation or CSRZ-DQPSK modulation having a characteristic of narrow spectrum (high frequency efficiency) have also been vigorously performed.
FIG. 12 is a diagram showing a configuration example of an optical transmitter and an optical receiver, which adopt a 43 Gbit/s RZ-DPSK or CSRZ-DPSK modulation format (hereinafter, referred to as (CS)RZ-DPSK modulation format). FIG. 13 is a diagram showing the state of optical power and optical phase when a (CS)RZ-DPSK modulated optical signal is transferred.
In FIG. 12, an optical transmitter 110 is for transmitting an optical signal in the 43 Gbit/s (CS)RZ-DPSK modulation format, and includes, for example, a transmission data processing section 111, a continuous wave (CW) light source 112, a phase modulator 113, and an RZ-pulsing intensity modulator 114.
Specifically, the transmission data processing section 111 has a function as a framer which frames input data, and a function as a forward error correction (FEC) encoder which adds an error-correcting code, and also has a function as a DPSK precoder which performs encoding in which difference information between a code one bit before and the current code is reflected.
The phase modulator 113 phase-modulates continuous light from the CW light source 112 in accordance with the encoding data from the transmission data processing section 111, and outputs an optical signal having constant optical intensity, but carrying information in binary optical phase, that is, a DPSK-modulated optical signal (see lower part in FIG. 13).
The RZ-pulsing intensity modulator 114 is for RZ-pulsing the optical signal from the phase modulator 113 (see upper part in FIG. 13). Particularly, an optical signal RZ-pulsed by using a clock drive signal having a frequency of the same bit rate as that of the data (43 GHz), and an amplitude of one times a driving voltage (Vπ) is referred to as an RZ-DPSK signal. Moreover, an optical signal RZ-pulsed by using a clock drive signal having a frequency of half the bit rate of the data (21.5 GHz), and an amplitude of two times the driving voltage (Vπ) is referred to as a CSRZ-DPSK signal.
An optical receiver 130 is connected to the optical transmitter 110 via a transmission path 120 and an optical repeater 121 for performing received signal processing with respect to the (CS)RZ-DPSK signal transmitted via optical repeater from the optical transmitter 110, and includes, for example, a delay interferometer 131, a photoelectric converter 132, a reproduction circuit 133, and a received data processing section 134.
Specifically, the delay interferometer 131 is formed of, for example, a Mach-Zehnder interferometer, and allows a delay component of one-bit time (23.3 ps in the configuration example of FIG. 1) for the (CS)RZ-DPSK signal, and a 0 rad phase-controlled component, to interfere (delay-interfere) with each other, and outputs the interference result as two outputs. The Mach-Zehnder interferometer is formed such that one of the branched waveguides becomes longer than the other by a propagation length corresponding to the one-bit time, and an electrode for phase-controlling the optical signal propagating in the other branched waveguide is formed.
The photoelectric converter 132 is formed from a dual-pin photo diode, which performs differential photoelectric conversion detection (balanced detection) by respectively receiving the outputs from the delay interferometer 131. The reception signal detected by the photoelectric converter 132 is appropriately amplified by an amplifier.
The reproduction circuit 133 is for extracting a data signal and a clock signal from the reception signal which has been differential photoelectric conversion-detected by the photoelectric converter 132. The received data processing section 134 is for performing signal processing such as error correction, based on the data signal and the clock signal extracted by the reproduction circuit 133.
FIG. 14 is a diagram showing a configuration example of an optical transmitter and an optical receiver, which adopt a 43 Gbit/s RZ-DQPSK or CSRZ-DQPSK modulation format (hereinafter, referred to as (CS)RZ-DQPSK modulation format). FIG. 15 is a diagram showing the state of optical power and optical phase when a (CS)RZ-DQPSK modulated optical signal is transferred. The configuration of an optical transmitter-receiver corresponding to the (CS)RZ-DQPSK modulation format is described in detail in, for example, Japanese Translation of PCT International Application, Publication No. 2004-516743, and hence only the outline thereof is described here.
In FIG. 14, an optical transmitter 210 includes, for example, a transmission data processing section 211, a 1:2 separating section (DEMUX) 212, a CW light source 213, a π/2 phase-shifter 214, two phase modulators 215A and 215B, and an RZ-pulsing intensity modulator 216.
Specifically, the transmission data processing section 211 has functions as the framer and the FEC encoder, as in the transmission data processing section 111 shown in FIG. 12, and a function as a DQPSK precoder which performs encoding in which difference information between a code one bit before and the current code is reflected.
The 1:2 separating section 212 is for separating 43 Gbit/s coded data from the transmission data processing section 211 into 21.5 Gbit/s two-sequence coded data #1 and #2.
The CW light source 213 is for outputting continuous light, and the output continuous light is separated into two, with one of the separated beams being input to the phase modulator 215A and the other being input to the phase modulator 215B via the π/2 phase shifter 214.
The phase modulator 215A modulates the continuous light from the CW light source 213 with the coded data #1 of one sequence separated by the 1:2 separating section 212, and outputs an optical signal carrying information in a binary optical phase (0 rad or π rad). To the phase modulator 215B are input beams obtained by phase-shifting the continuous light from the CW light source 213 by π/2 in the π/2 phase shifter 214, and the phase modulator 215B modulates the input beams with the coded data #2 of the other sequence separated by the 1:2 separating section 212, and outputs an optical signal carrying information in a binary optical phase (π/2 rad or 3π/2 rad). The beams modulated by the respective phase modulators 215A and 215B are multiplexed and output to the RZ-pulsing intensity modulator 216 on the subsequent stage. In other words, since the modulated beams from the respective phase modulators 215A and 215B are multiplexed, an optical signal having a constant optical intensity but carrying information in a four-valued optical phase (see lower part in FIG. 15), that is, a DQPSK-modulated optical signal is sent to the RZ-pulsing intensity modulator 216.
The RZ-pulsing intensity modulator 216 is for RZ-pulsing the DQPSK-modulated optical signal from the phase modulators 215A and 215B, as in the RZ-pulsing intensity modulator 114 shown in FIG. 12. Particularly, an optical signal RZ-pulsed by using a clock drive signal having a frequency of the same bit rate as that of the data #1 and #2, and an amplitude of one times the driving voltage (Vπ) is referred to as an RZ-DQPSK signal. Moreover, an optical signal RZ-pulsed by using a clock drive signal having a frequency of half the bit rate of the data #1 and #2 (10.75 GHz), and an amplitude of two times the driving voltage (Vπ) is referred to as a CSRZ-DQPSK signal.
Furthermore, an optical receiver 230 is connected to the optical transmitter 210 via a transmission path 220 and an optical repeater 221, for performing received signal processing with respect to the (CS)RZ-DQPSK signal transmitted via optical repeater from the optical transmitter 210, and includes, for example, a branching section 231 which branches the received optical signal into two, and further includes delay interferometers 232A and 232B, photoelectric converters 233A and 233B, and reproduction circuits 234A and 234B, respectively, on optical signal paths through which the branched optical signals propagate. Moreover, a 2:1 multiplexer 235 which multiplexes the data signals reproduced by the respective reproduction circuits 234A and 234B, and a received data processing section 236 are also provided.
Specifically, the optical signals obtained by branching the (CS)RZ-DQPSK signal transmitted through the transmission path 220 and the optical repeater 221, into two by the branching section 231 are respectively input to the respective delay interferometers 232A and 232B. The delay interferometer 232A allows a delay component of one-bit time (46.5 ps in the configuration example of FIG. 14), and a π/4 rad phase-controlled component, to interfere (delay-interfere) with each other, and outputs the interference result as two outputs. Moreover, the delay interferometer 232B allows a delay component of one-bit time, and a −π/4 rad phase-controlled component (the component the same as in the delay interferometer 232A is π/2 out of phase), to interfere (delay-interfere) with each other, and outputs the interference result as two outputs. Here, the respective delay interferometers 232A and 232B are respectively formed from a Mach-Zehnder interferometer, and the respective Mach-Zehnder interferometers are formed such that one of the branched waveguides becomes longer than the other by a propagation length corresponding to the one-bit time, and an electrode for phase-controlling the optical signal propagating in the other branched waveguide is formed.
The respective photoelectric converters 233A and 233B are formed from a dual-pin photo diode, which performs differential photoelectric conversion detection by respectively receiving the outputs from the respective photoelectric converters 233A and 233B. The reception signals detected by the respective photoelectric converters 233A and 233B are appropriately amplified by an amplifier.
The reproduction circuit 234A is for reproducing an in-phase component I relative to the clock signal and the data signal, from the received signal differential photoelectric conversion-detected in the photoelectric converter 233A. The reproduction circuit 234B is for reproducing a quadrature-phase component Q relative to the clock signal and the data signal, from the received signal differential photoelectric conversion-detected in the photoelectric converter 233B.
The in-phase component I and the quadrature-phase component Q are input to the 2:1 multiplexer 235 from the respective reproduction circuits 234A and 234B, and these components are converted to 43 Gbit/s data signals before the DQPSK modulation. The received data processing section 236 performs signal processing such as error correction based on the data signal from the 2:1 multiplexer 235.
Incidentally, the optical transmitters corresponding to a modulation format in which multivalue phase-modulated light is pulsed, such as the above described (CS) RZ-DPSK modulation format, or (CS)RZ-DQPSK modulation format (hereinafter, referred to as (CS)RZ-D(Q)PSK modulation format), both have a configuration in which a plurality of optical modulators are serially arranged. In such a modulation format using the plurality of optical modulators, there is a problem in that a change in the optical signal delay generated between the plurality of optical modulators may cause signal deterioration. As the conventional technique for dealing with this problem, for example, as shown in FIG. 16, a configuration has been proposed where phases of respective clock signals applied to a phase modulator 312 and an intensity modulator 313 sequentially connected to between a CW light source 311 and an output terminal, are compared with each other by a mixer 314, and an automatic delay compensation circuit (ADC) 315 controls the phase shift amount of a phase shifter 316 so that the phase relation between both clock signals becomes a constant value, based on the phase comparison result (for example, refer to Japanese Unexamined Patent Publication No. 2002-353896).
However, the above conventional technique is a method for directly monitoring drive signals applied to a plurality of modulators to detect a relative phase relation (delay difference), and performing feed-back control based on the detection result, and has a problem in that a delay shift in the optical level cannot be compensated, though a delay shift in the electrical level can be compensated.
Regarding the delay shift in the optical level, there is a problem in that, for example, as shown in FIG. 17, an optical propagation delay of a polarization maintaining fiber (PMF) 414 connecting between optical modulators 412 and 413 changes with temperature. FIG. 18 shows one example of a measurement result of a delay of the PMF in which a polyester elastomer is used as a fiber coating, relative to temperature change (reference temperature: 25° C.). Optical wavelength is assumed to be 1550 nm. It is seen from the measurement result shown in FIG. 18 that the delay increases with a rise in temperature.
FIG. 19 shows one example of a measurement result of a phase shift tolerance between drive signals (data/clock) of respective optical modulators on the transmission side, with respect to a system of the 43 Gbit/s RZ-DQPSK modulation format. When an allowable Q penalty is set to 0.2 dB, a phase shift tolerance width becomes 10 ps, which is a harsh value. Therefore, if a delay shift of the PMF occurs due to a temperature change as shown in FIG. 18, the delay shift due to the temperature change becomes a value which cannot be ignored, as compared with the tolerance, thereby causing signal deterioration.
Moreover, not only the temperature variant delay amount in the PMF for connecting the plurality of optical modulators, but also a temperature variant delay amount in an electronic circuit or an electric signal transmission path (for example, an electric coaxial cable) cause the signal deterioration. Furthermore, since the length of the PMF connecting the plurality of optical modulators changes according to the arrangement of the respective optical modulators and splice processing, there is another problem in that a delay shift occurs between respective drive signals. This problem cannot be solved by the conventional technique in which the delay shift between the drive signals applied to the respective optical modulators is directly monitored to perform feed-back control.
The present applicant has proposed a method in which optical spectrum of optical signals output from the optical modulators is monitored to perform feed-back control of phase shift between the drive signals based on an intensity change of a particular frequency component of the optical spectrum, with respect to optical modulators corresponding to the CS-RZ modulation format, though being different from the (CS)RZ-D(Q)PSK modulation format (for example, refer to Japanese Unexamined Patent Publication No. 2003-279912). According to the invention in the earlier application, by paying attention to the intensity change of the particular frequency component in the output light spectrum, a phase shift between signals of the driving system can be reliably detected, and the phase difference between the drive signals can be controlled so that an optimum driving condition can be stably obtained.
However, according to such a control method, the particular frequency component in the output light spectrum is extracted by using a narrow-band optical filter, to monitor the intensity change. However, if the extraction of the particular frequency component is not performed by using an optical filter having a sufficiently narrow bandwidth of a transparent band, there is a problem in that the monitoring accuracy of the intensity change decreases. For this problem, the present applicant has proposed a configuration in which an optical signal output from the optical modulator is photoelectrically converted to acquire an electrical spectrum, and a phase shift between the drive signals is determined based on an intensity change of the particular frequency component of the electrical spectrum (for example, refer to Japanese Unexamined Patent Publication No. 2004-294883). In the invention in the earlier application, for example, in the case where optical modulators on the previous stage and the post stage are driven by a 40 Gbit/s data signal and a 20 GHz clock signal, the phases of the data signal and the clock signal are feed-back controlled so that the power of the frequency component becomes maximum, by using the fact that the power of the frequency component over several GHz, centering on 25 GHz, of the electrical spectrum of the CS-RZ signal light output from the post-stage optical modulator decreases as the phase shift between the data signal and the clock signal increases.
However, the change in the electrical spectrum relative to the phase shift between drive signals corresponding to the abovementioned CS-RZ modulation format, and the change in the electrical spectrum relative to the phase shift between drive signals corresponding to the (CS)RZ-D(Q)PSK modulation format show different characteristics. Therefore, even if the control method in the invention in the earlier application is applied to the (CS)RZ-D(Q)PSK modulation format, it is difficult to effectively suppress waveform deterioration of the modulated light due to the phase shift between the drive signals. Accordingly, establishment of a new control method corresponding to the (CS)RZ-D(Q)PSK modulation format has been desired, which is considered to be promising as the next-generation modulation format excellent in the OSNR resistance and the nonlinearity resistance.